In a known fuel injector, a piezoelectric actuator is operable to control the delivery of fuel into a combustion space. The piezoelectric actuator typically includes a stack of piezoelectric elements across which a voltage is applied in use through an electrical connector. A fuel injector of this type is described in the Applicant's granted patent EP 0995901.
It is known to arrange the piezoelectric stack within an accumulator volume in the injector that is arranged, in use, to receive high-pressure fuel. Commonly, therefore, the piezoelectric actuator is immersed in fuel throughout its operational life. Moreover, when the fuel injection system is in use, the actuator is exposed to fuel at rail pressure ranging from, for example, 200 to 2000 bar. It is reasonable to expect an injector to have a lifetime of 15 years (>130000 hours). The fuel may be, for example, diesel, biodiesel, gasoline or mixtures optionally containing water and/or alcohol. It is also important to protect the piezoelectric actuator from environmental contaminants such as moisture. The presence of a fluid in the actuator can cause electrochemical effects and lead to a short circuit failure.
Thus, to protect the piezoelectric actuator from damage, it is important that the actuator is sealed so as to prevent fuel within the accumulator volume from entering the interior of the actuator and to prevent the ingress of fuel into the joints between the individual elements that form the actuator.
When the actuator is used in a hydrostatically pressurized mode, as described for example in the Applicant's granted patent EP 1096137, the piezoelectric actuator must be protected from the ingress of hydrostatic pressurised fluid.
The piezoelectric actuator can be provided with a coating or sleeve composed of a flexible sealant material, as described, for example, in EP 0995901. The coating helps to seal the elements of the piezoelectric actuator from fuel in the accumulator volume. Further, since the coating is flexible, the actuator is subjected to the compressive load applied by the fuel under pressure, helping to reduce the propagation of cracks in the structure. For this purpose, it is known to use an over-moulding technique to encapsulate the piezoelectric actuator within a plastics coating, or a sleeve member as described in the Applicant's granted patent application WO 02/061856. The coating or sleeve may also encapsulate the electrical connector, as described in the Applicant's granted patent EP 1079097. The sleeve is preferably formed from a low permeability fluoropolymer such as, for example, ethylene tetrafluoroethylene (ETFE), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), or polyvinylidene fluoride (PVDF).
It has been observed, however, that there may still be ingress of fluid into the interface between the actuator and the coating or sleeve. For example, the aforementioned sleeves are preferably formed from elastic or heat-shrink materials such as tubes with open first and second ends such that the actuator can be inserted into the open tube. The tube is then allowed to contract elastically, or is made to contract by the application of heat (i.e. heat shrinkage), in order to encapsulate the actuator. It will be understood that although the sleeve now bears tightly against the actuator, the first and second ends of the tube present paths for the ingress of fluid into the interface between the coating and the encapsulated actuator.
One or more clips may be provided at the ends of the sleeve in order to improve end sealing, as described in the Applicant's co-pending application EP 05256852. The clips are placed externally with respect to the sleeve and exert a constrictive force sufficient to urge the sleeve against the underlying external surface of the actuator so as to create a seal at the interface between the sleeve and the actuator surface. Nevertheless, under the challenging environmental conditions within a fuel injector there remains a risk that fluid may migrate underneath the ends of the sleeve despite the presence of the clips.
It is known to provide a filler material, as described, for example, in the Applicant's co-pending application WO 02/061856, between the coating and the encapsulated actuator in an attempt to limit the ingress of fluid between the coating and the actuator. However, this requires filling the entire space defined between the coating and the actuator with a curable substance, and may also require a degassing step, which adds to the complexity of the actuator arrangement and the method of assembly.
It is an object of the present invention to alleviate the above-mentioned problems but without adding significantly to the cost, complexity or size of the actuator arrangement.